1. Technical Field of the Invention
The present invention is directed to compositions of matter useful as high solids coatings and containing methylolamino curing agents.
2. Description of the Prior Art
High molecular weight acrylic and polyester resins have dominated the coatings industry, primarily as a result of the combination of properties, i.e., film hardness, flexibility, chemical resistance, water resistance, and gloss retention, required by the industry. However, with the advent of solvent emission regulations, the industry has turned to high solids coatings. Initial work on high solids coatings was directed to lowering the molecular weight of the acrylic and polyester resins. Because polyester resins could be made at much lower molecular weights than acrylic resins, most of the interest centered on making very low molecular weight polyester polyols which could be cured with melamines and other methylolamino curing agents.
Conventional high solids polyesters are made from short chained aliphatic diol and triols such as 1,4-butandiol, 1,6-hexandiol, trimethanol propane, or glycol ether polyols. These are reacted with difunctional acids or esters such as adipic acid, maleic anhydride, and phthalic anhydride. In order to get the proper combination of hardness and flexibility in low molecular weight polyesters it is important that a combination of hard and soft polyalcohols and polyacids be used. The hard components are usually the polyfunctional acids or esters such as phthalic anhydride. These are usually aromatic or unsaturated aliphatic acids which impart rigidity to the polyester resin, which translates into film hardness. The soft component is usually the polyol, e.g., 1,4-butandiol, 1,9-nonanediol, or glycol ether polyols. One exception is trimethanol propane which promotes film hardness by increasing crosslink density. If diols such as 1,6-hexandiol or 1,4-butandiol are used then unesterified or partially esterified very low molecular weight products in the polyester resin are volatilized when the polyester resin is cured with melamines. If glycol ether polyols are used such as ethylene or propylene glycol, then the ether functionality causes water sensitivity and poor gloss and color retention. Thus, while high solids polyesters have achieved some success in reducing solvent emissions in coatings, their performance properties are generally inferior to high molecular weight acrylic and polyester resins.
As used herein, the term methylolamino denotes a compound which is the product of an amino compound and an aldehyde (usually formaldehyde, giving rise to the methyl term) and the etherified and partially etherified derivatives thereof. Thus, the term encompasses polyfunctional amino compounds of the formula: EQU (R'HN).sub.n --R--NHR"
where n is 1 or greater, R' and R" are hydrogen or other groups including cycle forming carbons and R is a carbon containing backbone.
Methylolamino curing agents including those derived from melamines, guanamines, urea formaldehydes and glycoluril, are well known in the art. See, e.g., U.S. Pat. No. 4,246,376 to DiDomenico and the references cited therein, the disclosures of which are hereby incorporated by reference.